NITROGEN MINERALIZATION INDEXES IN FERROSOLS UNDER EUCALYPT PLANTATIONS OF NORTH-WESTERN TASMANIA - ASSOCIATION WITH PREVIOUS LAND-USE

Potentially mineralisable N was studied in the laboratory and in the h eld in ferrosols from eucalypt plantations and pastures in north-weste rn Tasmania. The plantation sites previously supported either Pinus ra diata plantations (ex-pine), mixed eucalypt-myrtle forest (ex-mixed), or open Eucalyptus delegatensis forest with a grass understory (ex-del e). Ranges of concentrations of organic C and total N, and C to N rati o, were 6.9-13.6%, 0.297-0.978%, and 10.3-30.4, respectively. The conc entration of organic C was independent of previous land use, but ranki ngs for total N, anaerobically mineralisable N, and extractable N in h ot KCl were pasture greater than or equal to ex-dele = ex-mixed greate r than or equal to ex-pine. The ratio C:N in theex-pine group (23.6) w as higher than ratios in other groups (13.1-15.6). Anaerobically miner alisable N ranged from 49 to 214 mu g/g, and was highly correlated wit h total N and extractable N in hot KCl at 95 degrees C for 16 h (r = 0 .83 for both cases, P < 0.001). The C:N ratio was negatively correlate d with total N and anaerobically mineralisable N (r = 0.56 and 0.70, P < 0.01). The results indicated that the pasture soils had greater ava ilability of mineral N, and that total N, C:N ratio, and anaerobically mineralisable N may provide useful information for assessing N minera lisation in these soils.